1. Field of the Invention
The present invention relates to a device for measuring altitude and barometric pressure and providing either altitude information or barometric pressure information by using electrical signal output from a pressure sensor.
2. Description of the Related Art
Recently electronic watches have become multi-functional, being able, for example to measure such things as atmospheric pressure or water pressure, temperature, etc., using a suitable sensor, and displaying that information in addition to normal watch functions such as time, alarm, or the like.
In U.S. Pat. No. 4,783,772, a wristwatch having a water pressure indicating function is disclosed, the main construction thereof relating to a circuit for converting data output from a pressure sensor unit to a pressure value, and the construction of the circuit, the pressure sensor, and the displaying device. This patent also refers to use of this wristwatch as an altitude indicator, although there is no indication how it can be used as an altitude indicator, and no disclosure of technology for measuring altitude with suitable compensation for precisely indicating altitude at any location.
The assignee of this invention had previously proposed the idea of a battery powered device for processing a signal received from a sensor, in the specification of U.S. patent application Ser. No. 07/168,728, now U.S. Pat. No. 4,879,699.
In accordance with this application, the sensor signal processing device is constructed by a sensor signal processing apparatus comprising: a power source unit; a sensor for detecting physical information; a sensor driving circuit for driving the sensor; an analog signal processor for inputting and processing a sensor signal output from said sensor; an A/D converter for converting the sensor signal processed by said analog signal processor into digital data; a data processor for generating sensor data from the digital data output from said A/D converter; and a controlling signal generator for generating control signals for controlling operations of each circuit; and the analog signal processor, being characterized in that the analog signal processor includes a sample-and-hold circuit for sampling-and-holding the sensor signal and supplying the signal to said A/D converter, and said controlling signal generator generates a control signal for operating said sensor driver and said A/D converter at different timing and operating said sample-and-hold circuit within a driving period of said sensor driver.
In the application, a small portable electronic device, such as an electric watch, provided with a function for measuring only an barometric pressure, is disclosed as an example. However, the technology for measuring altitude is not disclosed.
This kind of portable barometric pressure measuring device is used frequently for mountain climbing so it would be very convenient if not only the pressure but also altitude could be measured simultaneously. The present invention was created in response to this requirement; and an object of the present invention is to provide a barometric pressure and altitude measuring device capable of displaying altitude and/or pressure information by using a sensor and circuit commonly used for measuring pressure, without greatly increasing the production cost.
Another object of the present invention is to provide a precision altitude measuring device with a reduce error factor caused by the variation in temperature at sea level.
Still another object of the present invention is to provide a wristwatch having an altitude display capability.
It is apparent that altitude cannot accurately be measured using only a conventional pressure sensor. A conventional portable altitude measuring device generally measures barometric pressure using an Aneroid Barometer and displays the altitude by converting the measured barometric pressure into altitude based on the standard atmosphere defined as a pressure of 1013.25 mb at sea level at temperature of 15.degree. C. In this method, a suitable mechanical gauge can be used to display the altitude measured as explained above.
However, the above method of measuring and displaying altitude information is susceptible to error caused by changes in temperature and barometric pressure.
Accordingly, when the above type of altitude measuring device is used for mountain climbing or the like, it is necessary to adjust the device at some known altitude in order to reduce the error caused by changes in temperature or barometric pressure.
A standard unit of barometric Pressure, known as an atmosphere, has been adopted internationally. The international standard barometric or ICAO standard atmosphere was adopted by the International Civil Aviation Organization in 1964, and shows the relationship between altitude, barometric pressure, and temperature as follows: EQU H=153.85.times.T.sub.0 .times.[1-(P/P.sub.0).sup.0.190255 ](1)
Wherein, T.sub.0 =288.15 K., P=the barometric pressure at the location where the measurement is carried out and P.sub.0 =1013.25 mbar at sea level, and wherein K is the Kelvin temperature scale and 288.15 K. corresponds to 15.0.degree. C.
The equation (1) above is defined at a temperature variation rate of -6.5.degree. C./1000 m, i.e., a 6.5.degree. C. reduction in temperature for every 1000 m increase in altitude. In equation (1), T.sub.0 represents the temperature of the atmosphere at sea level, and P.sub.0 represents the barometric pressure at sea level. As is apparent from equation (1), the relationship between temperature and barometric pressure will vary with respect to altitude. When the above altitude measuring device is actually used, the temperature and the pressure at sea level may of course vary from 15.0.degree. C. and 1013.25 mbar of the standard atmosphere, and moreover, the temperature variation rate in different regions, seasonal conditions, climate, latitude, or the like make it very difficult to accurately measure altitude using the above method, and it is considered necessary to compensate the altitude measurement thus obtained at some known altitude.
For example, when the temperature at sea level is 15.0.degree. C. and the pressure is 1030 mbar, which is not the standard atmosphere while the actual pressure at 1000 m altitude above sea level will be 913.5 mbar, the altitude measured utilizing the relationship between the barometric pressure and the altitude based on the standard atmosphere will show an altitude of 865 m, while under the same conditions the pressure of 859.5 mbar at an altitude of 1500 m above sea level will lead to an altitude measurement of 1365 m.
That indicates that an altitude measurement taken at an altitude of 1000 m or 1500 m will have an error of -135 m.
If the compensation material above is made at some location known to be exactly 1000 m above sea level then a subsequent measurement at an altitude of 1500 m, will show 1500 m (1365 m+135m=1500m).
In the same way, when the barometric pressure at sea level is 980 mbar, an altitude measurement taken at 1000 m above sea level will indicate 1274 m, and a measurement taken at an altitude of 1500 m will indicate 1771 m. If a compensation of -274 m is made at a location known to be 1000 m, an altitude measurement taken at an altitude of 1500 m will indicate 1497 m (1771m-274m=1497m), reducing the error into only -3 m.
Thus the error in altitude measurement caused by variations in barometric pressure at sea level can be reduced to a point of being negligible.
However, another problem with the prior art measurement method, is apparent from equation (1). Large errors in altitude measurement can result from variations in the temperature of the atmosphere at sea level. The most significant error factor in equation (1) when measuring altitude is the temperature T.sub.0 at sea level. The error resulting when the temperature varies will be larger than that caused by variations in pressure.
That means, even when the error is compensated at a known reference place to adjust the altitude indicated by the altitude measuring device utilizing the above equation to the actual altitude, an error will occur in proportion to the error of the absolute temperature at sea level, with respect to the relative altitude, which is either higher or lower than that of the place at which the adjustment is being carried out.
For example, when the barometric pressure is 1013.25 mbar and the temperature is 0.degree. C. at sea level, an altitude measurement will indicate 1055 m at an actual altitude of 1000 m above sea level, and 1582 m at an actual altitude of 1500 m above sea level.
Even it a compensation of -55 m is made at a known altitude of 1000 m, the device will still indicate 1527 m at an altitude of 1500 m resulting in an error of 27 m.
As can be seen from the above explanation, if the pressure at sea level is known, a more precise altitude measurement can be made, and when both the temperature and the pressure at sea level are known, a much more precise altitude measurement can be made.